He might have had a plot in mind but not all the scripts at once. If he had. Then I doubt there would be as many plot holes as they are. Still they are good movies, though in my opinion I believe the OT to be the best.

Even though it is not strictly 100% canon - the novelisation of ROTS states that the Jedi had become stagnant and that is why the Sith won the day. The sith had evolved, learned, changed and developed. The Jedi were the same Jedi, still thought, acted, and made the same Jedi decisions.

The choice not to train from birth was 2 folds

1. to keep them hidden until time - DER

2. So that Luke could live a life like his Father's and therefore be in a category to best him. Living this life would give him the confidence and the assurance that what he was doing was right and not get confused like dear ol dad.

Yoda could have trained Luke and Leia at a very young age on Dagobah and they would grow up learning from the best and been Jedi Masters by the time they met Darth Vader. Dagobah was the perfect place to train a powerful force sensitive without raising any suspicions from the Emperor or Vader because the whole planet was consumed in dark side energy. and by them using the force there wouldnt effect the force signal they produced. They would reach their full potential in 20 years and would be ready to take on Vader and the Palpatine together and win. However Vader would have died a slave to the dark side and you wouldnt see his force ghost at the end.